Understanding SNMP

SNMP is an application-layer protocol that provides a message format for communication between managers and agents. The SNMP system consists of an SNMP manager, an SNMP agent, and a MIB. The SNMP manager can be part of a network management system (NMS) such as CiscoWorks. The agent and MIB reside on the switch. To configure SNMP on the switch, you define the relationship between the manager and the agent.

The SNMP agent contains MIB variables whose values the SNMP manager can request or change. A manager can get a value from an agent or store a value into the agent. The agent gathers data from the MIB, the repository for information about device parameters and network data. The agent can also respond to a manager’s requests to get or set data.

An agent can send unsolicited traps to the manager. Traps are messages alerting the SNMP manager to a condition on the network. Traps can mean improper user authentication, restarts, link status (up or down), MAC address tracking, closing of a TCP connection, loss of connection to a neighbor, or other significant events.

SNMP Versions

SNMPv2C replaces the Party-based Administrative and Security Framework of SNMPv2Classic with the community-string-based Administrative Framework of SNMPv2C while retaining the bulk retrieval and improved error handling of SNMPv2Classic. It has these features:

SNMPv3—Version 3 of the SNMP is an interoperable standards-based protocol defined in RFCs 2273 to 2275. SNMPv3 provides secure access to devices by authenticating and encrypting packets over the network and includes these security features:

– Message integrity—ensuring that a packet was not tampered with in transit

– Authentication—determining that the message is from a valid source

– Encryption—mixing the contents of a package to prevent it from being read by an unauthorized source.

Note To select encryption, enter the priv keyword. This keyword is available only when the cryptographic (encrypted) software image is installed.

Both SNMPv1 and SNMPv2C use a community-based form of security. The community of managers able to access the agent’s MIB is defined by an IP address access control list and password.

SNMPv2C includes a bulk retrieval mechanism and more detailed error message reporting to management stations. The bulk retrieval mechanism retrieves tables and large quantities of information, minimizing the number of round-trips required. The SNMPv2C improved error-handling includes expanded error codes that distinguish different kinds of error conditions; these conditions are reported through a single error code in SNMPv1. Error return codes in SNMPv2C report the error type.

SNMPv3 provides for both security models and security levels. A security model is an authentication strategy set up for a user and the group within which the user resides. A security level is the permitted level of security within a security model. A combination of the security level and the security model determine which security mechanism is used when handling an SNMP packet. Available security models are SNMPv1, SNMPv2C, and SNMPv3.

Table 35-1 identifies the characteristics of the different combinations of security models and levels.

Table 35-1 SNMP Security Models and Levels

Model

Level

Authentication

Encryption

Result

SNMPv1

noAuthNoPriv

Community string

No

Uses a community string match for authentication.

SNMPv2C

noAuthNoPriv

Community string

No

Uses a community string match for authentication.

SNMPv3

noAuthNoPriv

Username

No

Uses a username match for authentication.

SNMPv3

authNoPriv

Message Digest 5 (MD5) or Secure Hash Algorithm (SHA)

No

Provides authentication based on the HMAC-MD5 or HMAC-SHA algorithms.

SNMPv3

authPriv (requires the cryptographic software image)

MD5 or SHA

Data Encryption Standard (DES) or Advanced Encryption Standard (AES)

Provides authentication based on the HMAC-MD5 or HMAC-SHA algorithms. Allows specifying the User-based Security Model (USM) with these encryption algorithms:

DES 56-bit encryption in addition to authentication based on the CBC-DES (DES-56) standard.

3DES 168-bit encryption

AES 128-bit, 192-bit, or 256-bit encryption

You must configure the SNMP agent to use the SNMP version supported by the management station. Because an agent can communicate with multiple managers, you can configure the software to support communications using SNMPv1, SNMPv2C, or SNMPv3.

SNMP Manager Functions

The SNMP manager uses information in the MIB to perform the operations described in Table 35-2.

Retrieves large blocks of data, such as multiple rows in a table, that would otherwise require the transmission of many small blocks of data.

get-response

Replies to a get-request, get-next-request, and set-request sent by an NMS.

set-request

Stores a value in a specific variable.

trap

An unsolicited message sent by an SNMP agent to an SNMP manager when some event has occurred.

1.With this operation, an SNMP manager does not need to know the exact variable name. A sequential search is performed to find the needed variable from within a table.

2.The get-bulk command only works with SNMPv2 or later.

SNMP Agent Functions

The SNMP agent responds to SNMP manager requests as follows:

Get a MIB variable—The SNMP agent begins this function in response to a request from the NMS. The agent retrieves the value of the requested MIB variable and responds to the NMS with that value.

Set a MIB variable—The SNMP agent begins this function in response to a message from the NMS. The SNMP agent changes the value of the MIB variable to the value requested by the NMS.

The SNMP agent also sends unsolicited trap messages to notify an NMS that a significant event has occurred on the agent. Examples of trap conditions include, but are not limited to, when a port or module goes up or down, when spanning-tree topology changes occur, and when authentication failures occur.

SNMP Community Strings

SNMP community strings authenticate access to MIB objects and function as embedded passwords. In order for the NMS to access the switch, the community string definitions on the NMS must match at least one of the three community string definitions on the switch.

A community string can have one of these attributes:

Read-only (RO)—Gives read access to authorized management stations to all objects in the MIB except the community strings, but does not allow write access

Read-write (RW)—Gives read and write access to authorized management stations to all objects in the MIB, but does not allow access to the community strings

When a cluster is created, the command switch manages the exchange of messages among member switches and the SNMP application. The Network Assistant software appends the member switch number ( @esN, where N is the switch number) to the first configured RW and RO community strings on the command switch and propagates them to the member switches. For more information, see Chapter 6, “Clustering Switches” and see Getting Started with Cisco Network Assistant, available on Cisco.com.

Using SNMP to Access MIB Variables

An example of an NMS is the CiscoWorks network management software. CiscoWorks 2000 software uses the switch MIB variables to set device variables and to poll devices on the network for specific information. The results of a poll can be displayed as a graph and analyzed to troubleshoot internetworking problems, increase network performance, verify the configuration of devices, monitor traffic loads, and more.

As shown in Figure 35-1, the SNMP agent gathers data from the MIB. The agent can send traps, or notification of certain events, to the SNMP manager, which receives and processes the traps. Traps alert the SNMP manager to a condition on the network such as improper user authentication, restarts, link status (up or down), MAC address tracking, and so forth. The SNMP agent also responds to MIB-related queries sent by the SNMP manager in get-request, get-next-request, and set-request format.

Figure 35-1 SNMP Network

SNMP Notifications

SNMP allows the switch to send notifications to SNMP managers when particular events occur. SNMP notifications can be sent as traps or inform requests. In command syntax, unless there is an option in the command to select either traps or informs, the keyword traps refers to either traps or informs, or both. Use the snmp-server host command to specify whether to send SNMP notifications as traps or informs.

Note SNMPv1 does not support informs.

Traps are unreliable because the receiver does not send an acknowledgment when it receives a trap, and the sender cannot determine if the trap was received. When an SNMP manager receives an inform request, it acknowledges the message with an SNMP response protocol data unit (PDU). If the sender does not receive a response, the inform request can be sent again. Because they can be re-sent, informs are more likely than traps to reach their intended destination.

The characteristics that make informs more reliable than traps also consume more resources in the switch and in the network. Unlike a trap, which is discarded as soon as it is sent, an inform request is held in memory until a response is received or the request times out. Traps are sent only once, but an inform might be re-sent or retried several times. The retries increase traffic and contribute to a higher overhead on the network. Therefore, traps and informs require a trade-off between reliability and resources. If it is important that the SNMP manager receive every notification, use inform requests. If traffic on the network or memory in the switch is a concern and notification is not required, use traps.

SNMP ifIndex MIB Object Values

In an NMS, the IF-MIB generates and assigns an interface index (ifIndex) object value that is a unique number greater than zero to identify a physical or a logical interface. When the switch reboots or the switch software is upgraded, the switch uses this same value for the interface. For example, if the switch assigns a port 2 an ifIndex value of 10003, this value is the same after the switch reboots.

The switch uses one of the values in Table 35-3 to assign an ifIndex value to an interface:

If no keyword is entered, the default is the noauth (noAuthNoPriv) security level.

SNMP notification type

If no type is specified, all notifications are sent.

5.This is the default when the switch starts and the startup configuration does not have any snmp-server global configuration commands.

SNMP Configuration Guidelines

If the switch starts and the switch startup configuration has at least one snmp-server global configuration command, the SNMP agent is enabled.

An SNMP group is a table that maps SNMP users to SNMP views. An SNMP user is a member of an SNMP group. An SNMP host is the recipient of an SNMP trap operation. An SNMP engine ID is a name for the local or remote SNMP engine.

When configuring SNMP, follow these guidelines:

When configuring an SNMP group, do not specify a notify view. The snmp-server host global configuration command autogenerates a notify view for the user and then adds it to the group associated with that user. Modifying the group's notify view affects all users associated with that group. See the Cisco IOS Network Management Command Reference for information about when you should configure notify views.

To configure a remote user, specify the IP address or port number for the remote SNMP agent of the device where the user resides.

Before you configure remote users for a particular agent, configure the SNMP engine ID, using the snmp-server engineID global configuration with the remote option. The remote agent's SNMP engine ID and user password are used to compute the authentication and privacy digests. If you do not configure the remote engine ID first, the configuration command fails.

When configuring SNMP informs, you need to configure the SNMP engine ID for the remote agent in the SNMP database before you can send proxy requests or informs to it.

If a local user is not associated with a remote host, the switch does not send informs for the auth (authNoPriv) and the priv (authPriv) authentication levels.

Changing the value of the SNMP engine ID has important side effects. A user's password (entered on the command line) is converted to an MD5 or SHA security digest based on the password and the local engine ID. The command-line password is then destroyed, as required by RFC 2274. Because of this deletion, if the value of the engine ID changes, the security digests of SNMPv3 users become invalid, and you need to reconfigure SNMP users by using the snmp-server user username global configuration command. Similar restrictions require the reconfiguration of community strings when the engine ID changes.

The snmp-server inform retries number timeout seconds pending number global configuration command is used to specify the retry options for the SNMP server. The retry interval is exponential, and is calculated as 2^(retry number)×(retry timer).

Disabling the SNMP Agent

Beginning in privileged EXEC mode, follow these steps to disable the SNMP agent:

Command

Purpose

Step 1

configure terminal

Enter global configuration mode.

Step 2

no snmp-server

Disable the SNMP agent operation.

Step 3

end

Return to privileged EXEC mode.

Step 4

show running-config

Verify your entries.

Step 5

copy running-config startup-config

(Optional) Save your entries in the configuration file.

The no snmp-server global configuration command disables all running versions (Version 1, Version 2C, and Version 3) on the device. No specific Cisco IOS command exists to enable SNMP. The first snmp-server global configuration command that you enter enables all versions of SNMP.

Configuring Community Strings

You use the SNMP community string to define the relationship between the SNMP manager and the agent. The community string acts like a password to permit access to the agent on the switch. Optionally, you can specify one or more of these characteristics associated with the string:

An access list of IP addresses of the SNMP managers that are permitted to use the community string to gain access to the agent

A MIB view, which defines the subset of all MIB objects accessible to the given community

Read and write or read-only permission for the MIB objects accessible to the community

Beginning in privileged EXEC mode, follow these steps to configure a community string on the switch:

Note The @ symbol is used for delimiting the context information. Avoid using the @ symbol as part of the SNMP community string when configuring this command.

For string, specify a string that acts like a password and permits access to the SNMP protocol. You can configure one or more community strings of any length.

(Optional) For view, specify the view record accessible to the community.

(Optional) Specify either read-only (ro) if you want authorized management stations to retrieve MIB objects, or specify read-write (rw) if you want authorized management stations to retrieve and modify MIB objects. By default, the community string permits read-only access to all objects.

(Optional) For access-list-number, enter an IP standard access list numbered from 1 to 99 and 1300 to 1999.

(Optional) If you specified an IP standard access list number in Step 2, then create the list, repeating the command as many times as necessary.

For access-list-number, enter the access list number specified in Step 2.

The deny keyword denies access if the conditions are matched. The permit keyword permits access if the conditions are matched.

For source, enter the IP address of the SNMP managers that are permitted to use the community string to gain access to the agent.

(Optional) For source-wildcard, enter the wildcard bits in dotted decimal notation to be applied to the source. Place ones in the bit positions that you want to ignore.

Recall that the access list is always terminated by an implicit deny statement for everything.

Step 4

end

Return to privileged EXEC mode.

Step 5

show running-config

Verify your entries.

Step 6

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Note To disable access for an SNMP community, set the community string for that community to the null string (do not enter a value for the community string).

To remove a specific community string, use the no snmp-server community string global configuration command.

This example shows how to assign the string comaccess to SNMP, to allow read-only access, and to specify that IP access list 4 can use the community string to gain access to the switch SNMP agent:

Switch(config)# snmp-server community comaccess ro 4

Configuring SNMP Groups and Users

You can specify an identification name (engine ID) for the local or remote SNMP server engine on the switch. You can configure an SNMP server group that maps SNMP users to SNMP views, and you can add new users to the SNMP group.

Beginning in privileged EXEC mode, follow these steps to configure SNMP on the switch:

The engineid-string is a 24-character ID string with the name of the copy of SNMP. You need not specify the entire 24-character engine ID if it has trailing zeros. Specify only the portion of the engine ID up to the point where only zeros remain in the value. For example, to configure an engine ID of 123400000000000000000000, you can enter this: snmp-server engineID local 1234

If you select remote, specify the ip-address of the device that contains the remote copy of SNMP and the optional User Datagram Protocol (UDP) port on the remote device. The default is 162.

The username is the name of the user on the host that connects to the agent.

The groupname is the name of the group to which the user is associated.

Enter remote to specify a remote SNMP entity to which the user belongs and the hostname or IP address of that entity with the optional UDP port number. The default is 162.

Enter the SNMP version number (v1, v2c, or v3). If you enter v3, you have these additional options:

– encrypted specifies that the password appears in encrypted format. This keyword is available only when the v3 keyword is specified.

– auth is an authentication level setting session that can be either the HMAC-MD5-96 (md5) or the HMAC-SHA-96 (sha) authentication level and requires a password string auth-password (not to exceed 64 characters).

If you enter v3 and the switch is running the cryptographic software image, you can also configure a private (priv) encryption algorithm and password string priv-password (not to exceed 64 characters).

– priv specifies the User-based Security Model (USM).

– des specifies the use of the 56-bit DES algorithm.

– 3des specifies the use of the 168-bit DES algorithm.

– aes specifies the use of the DES algorithm. You must select either 128-bit, 192-bit, or 256-bit encryption.

(Optional) Enter access access-list with a string (not to exceed 64 characters) that is the name of the access list.

Configuring SNMP Notifications

A trap manager is a management station that receives and processes traps. Traps are system alerts that the switch generates when certain events occur. By default, no trap manager is defined, and no traps are sent. Switches running this Cisco IOS release can have an unlimited number of trap managers.

Note Many commands use the word traps in the command syntax. Unless there is an option in the command to select either traps or informs, the keyword traps refers to traps, informs, or both. Use the snmp-server host global configuration command to specify whether to send SNMP notifications as traps or informs.

Table 35-5 describes the supported switch traps (notification types). You can enable any or all of these traps and configure a trap manager to receive them. To enable the sending of SNMP inform notifications, use the snmp-server enable traps global configuration command combined with the snmp-server host host-addr informs global configuration command.

Table 35-5 Switch Notification Types

Notification Type Keyword

Description

bgp

Generates Border Gateway Protocol (BGP) state change traps. This option is only available when the enhanced multilayer image is installed.

bridge

Generates STP bridge MIB traps.

cluster

Generates a trap when the cluster configuration changes.

config

Generates a trap for SNMP configuration changes.

copy-config

Generates a trap for SNMP copy configuration changes.

entity

Generates a trap for SNMP entity changes.

cpu threshold

Allow CPU-related traps.

envmon

Generates environmental monitor traps. You can enable any or all of these environmental traps: fan, shutdown, status, supply, temperature.

errdisable

Generates a trap for a port VLAN errdisabled. You can also set a maximum trap rate per minute. The range is from 0 to 10000; the default is 0, which means there is no rate limit.

The snmp-server host command specifies which hosts receive the notifications. The snmp-server enable trap command globally enables the mechanism for the specified notification (for traps and informs). To enable a host to receive an inform, you must configure an snmp-server host informs command for the host and globally enable informs by using the snmp-server enable traps command.

To remove the specified host from receiving traps, use the no snmp-server host host global configuration command. The no snmp-server host command with no keywords disables traps, but not informs, to the host. To disable informs, use the no snmp-server host informs global configuration command. To disable a specific trap type, use the no snmp-server enable traps notification-types global configuration command.

Setting the CPU Threshold Notification Types and Values

Beginning in privileged EXEC mode, follow these steps to set the CPU threshold notification types and values:

Create a standard access list, repeating the command as many times as necessary.

For access-list-number, enter the access list number specified in Step 2.

The deny keyword denies access if the conditions are matched. The permit keyword permits access if the conditions are matched.

For source, enter the IP address of the TFTP servers that can access the switch.

(Optional) For source-wildcard, enter the wildcard bits, in dotted decimal notation, to be applied to the source. Place ones in the bit positions that you want to ignore.

Recall that the access list is always terminated by an implicit deny statement for everything.

Step 4

end

Return to privileged EXEC mode.

Step 5

show running-config

Verify your entries.

Step 6

copy running-config startup-config

(Optional) Save your entries in the configuration file.

SNMP Examples

This example shows how to enable all versions of SNMP. The configuration permits any SNMP manager to access all objects with read-only permissions using the community string public. This configuration does not cause the switch to send any traps.

Switch(config)# snmp-server community public

This example shows how to permit any SNMP manager to access all objects with read-only permission using the community string public. The switch also sends VTP traps to the hosts 192.180.1.111 and 192.180.1.33 using SNMPv1 and to the host 192.180.1.27 using SNMPv2C. The community string public is sent with the traps.

This example shows how to allow read-only access for all objects to members of access list 4 that use the comaccess community string. No other SNMP managers have access to any objects. SNMP Authentication Failure traps are sent by SNMPv2C to the host cisco.com using the community string public.

This example shows how to send Entity MIB traps to the host cisco.com. The community string is restricted. The first line enables the switch to send Entity MIB traps in addition to any traps previously enabled. The second line specifies the destination of these traps and overwrites any previous snmp-server host commands for the host cisco.com.

Displaying SNMP Status

To display SNMP input and output statistics, including the number of illegal community string entries, errors, and requested variables, use the show snmp privileged EXEC command. You also can use the other privileged EXEC commands in Table 35-6 to display SNMP information. For information about the fields in the displays, see the Cisco IOS Configuration Fundamentals Command Reference.

Table 35-6 Commands for Displaying SNMP Information

Feature

Default Setting

show snmp

Displays SNMP statistics.

show snmp engineID [ local | remote ]

Displays information on the local SNMP engine and all remote engines that have been configured on the device.

show snmp group

Displays information on each SNMP group on the network.

show snmp pending

Displays information on pending SNMP requests.

show snmp sessions

Displays information on the current SNMP sessions.

show snmp user

Displays information on each SNMP user name in the SNMP users table.

Note You must use this command to display SNMPv3 configuration information for auth | noauth | priv mode. This information is not displayed in the show running-config output.